The invention relates to metal plating systems, and in particular to plating systems using a cyanide solution as a plating bath. Specifically, the invention relates to a method and apparatus for the removal of sodium carbonate from a cyanide-zinc plating bath solution.
It is sometimes necessary to remove carbonates that build up in alkaline-cyanide plating baths as a result of absorption of carbon dioxide (CO.sub.2) from the surrounding air and from cyanide decomposition at the anode of the plating system.
When excessive carbonates build up in the plating baths, rough or dull deposits on the plated surface may result.
There is an on-going emphasis on reducing the amount of chemical agents entering the waste effluent. As a result, there is a corresponding emphasis on reducing the dragout losses in plating operations, and also an emphasis on recycling dragout chemicals back into the plating bath solutions. This action results in a tendency for the carbonate content of plating baths to become higher than is generally normal. Under the circumstances, it is necessary to effect the removal of the carbonate compound more often. This invention provides a method and apparatus to accomplish that removal conveniently and efficiently.
The conventional method of removing carbonate compounds in each year, during the winter, is to transfer the plating bath solution to another tank outside and at or near freezing temperatures. The sodium carbonate then precipitates from the plating bath solution and settles to the bottom of the tank. The supernatant liquid is then pumped back to the plating tank.
There are disadvantages to the conventional method. There is poor control of the temperature and much loss of desirable bath constituents may result from too low a temperature. Bath solution may be trapped along with the carbonate crystals. Another disadvantage is that not all plating facilities have the necessary equipment and extra tanks to utilize this method. In some areas there may not be a cold weather climate to precipitate the carbonate compounds in this manner.
Another method used for removing carbonate compounds is to add barium or calcium salts which precipitate the carbonate compounds. This method also requires an extra tank and is troublesome. This method also can change the plating bath composition if the barium or calcium salts are not added with proper anion or in the proper quantities.
It is to be understood that only the sodium-based plating bath solutions can be sodium carbonate precipitated by both the cooling method and the chemical precipitation method. The potassium plating bath solutions produce such soluable carbonate solutions that only the chemical precipitation method can be used.
The method and apparatus of this invention for the removal of carbonate from a cyanide-zinc plating bath solution involves the use of a container, open at one end, dry ice, and water. The container is submerged in the plating bath solution with the open end extending above the level of the plating bath solution. The container is filled with dry ice and water which produces a temperature of approximately zero degrees centigrade inside the container.
The plating bath solution adjacent to the exterior of the container is cooled so that excess sodium carbonate is precipitated as a crystalline deposit on the exterior of the container. After a desired interval, the container is removed from the plating bath solution with the encrustation of sodium carbonate for disposal.
It is to be understood that to fill the container with the dry ice and water before submerging the container into the plating bath solution is within the scope and intent of the invention.
Experimentation with a cylindrical container four inches in diameter and 30 inches long proved the feasibility of the method and apparatus. Cup-like appendages affixed to the exterior of the cylinder were used to catch any precipitated crystals that might fall off, however there was little evidence of this. The cup-like appendages also served to prevent the deposit of precipitated crystals from sliding off of the cylindrical container before removing it from the plating bath solution.
In order to further lower the temperature of the liquid in the container, a water-miscible solvent may be added to the liquid in the container, such as a low molecular weight alcohol or acetone.
The greater the exterior surface of the container the greater the amount of carbonate that can be removed. For example, a thin rectangular container approximately only one and one-half inches wide, but of long length and height, would accept cakes or slabs of dry ice (which are usually about one inch thick and approximately ten inches wide by ten inches long). Another means of increasing the exterior surface is to corrugate the exterior "skin" of the container. Still another means is to extend fins from the container exterior surface that would conduct the cold temperature from the main container for some distance from the container, and thus provide more surface on which to precipitate the carbonate crystals.
It is to be understood that the aforementioned variations in configuration and other geometrical and mechanical variations to increase the exterior surface area are within the scope and intent of the method and apparatus of this invention.
A variation in the shape of the exterior surface, such as by a series of corrugations, will aid in preventing the encrusted crystals from sliding off of the container. Such configuration eliminates the necessity of the aforementioned cup-like appendages.
An analysis of a sample of the dry encrustation removed from a container indicates that there was a ratio of 58 parts of sodium carbonate to 0.03 part of sodium cyanide and 0.002 part of zinc. Thus, there was little loss of cyanide or zinc.
It is, therefore an object of the invention to provide a method and apparatus to remove carbonate compounds from plating bath solutions.
It is another object of the invention to provide a method and apparatus to remove carbonate compounds from plating bath solutions with very little loss of the main constituents of the plating bath solution.
It is also an object of the invention to provide an apparatus to remove carbonate compounds from plating bath solutions that is simple in structure.
It is still another object of the invention to provide a method to remove carbonate compounds from plating bath solutions that is simple in operation.
It is yet another object of the invention to provide a method and apparatus to remove carbonate compounds from plating bath solutions that does not require extra equipment and extra tanks to transfer the plating bath solution for the operation.
It is also still another object of the invention to provide a method and apparatus to remove carbonate compounds from plating both solutions that can be used in any area regardless of weather conditions.
Further objects and advantages of the invention will become more apparent in the light of the following description of the preferred embodiments.